Thanks for your question.
The two unchangeable things are God's word (ie, the promise itself) and the vow that He added to it.
God's word, of course, is of absolute validity in itself; the vow, therefore, is superfluous, but exactly because of this, exceptionally impressive. It is impossible, then, that God could have lied (Num 23:19; 1Sa 15:29; John 7:18; Tit 1:2); He is doubly bound to be faithful to His promises to Abraham. The result is that we may very courageously hold the hope offered to us. The implication is that the Christian hope consists of nothing other than what God promised to Abraham (cp Rom 15:8) and therefore that our realization of that hope is finally as certain as God's word and his oath. The unity between old and new is in view.
If you want to come to grips with what the Bible really teaches, why not take the free online Bible study course on our website, https://www.thisisyourbible.com? This course will give you a background in the major themes of the Bible from Genesis to Revelation. As with everything we offer, there is no cost to you other than your time and effort. You will also have a personal tutor to whom you may pose questions either from the course itself or those questions which come to you as you read the Bible.
I hope you have found this helpful.